This invention relates generally to improvements in electron projection systems, and, more particularly, to new and improved apparatus for supporting and securing target wafers for exposure to an electron pattern or beam in such systems.
Electron beam pattern generation systems, in which a pattern of electron radiation on a sensitized surface is generated by movement of a single focused beam, and electron image projection systems, in which the pattern of electron radiation is emitted from a cathode mask, are both well known in the art. In either type of system, an electron-sensitized surface of a wafer of target material is exposed to the desired electron pattern, and, in the manufacture of micro-electronic circuitry, is subsequently developed, etched, and further processed to produce a completed integrated circuit. Masks used in integrated circuit fabrication may themselves be generated using an electron beam pattern generator, or may be fabricated from a master mask by utilizing an electron image projection system. Although both image projection systems and beam pattern generators of the general type described are known in the prior art, they have so far enjoyed only limited use in the fabrication of micro-electronic circuitry because of certain limitations, one of which is overcome by the present invention. In order to fully appreciate these limitations, it is desirable to have at least a basic understanding of the principles of operation of electron image projection systems and electron beam pattern generators. Accordingly, the following background material summarizes the prior art systems and their limitations.
An electron pattern generator includes an electron gun, together with a conventional electron beam deflection system. The deflection system may be controlled by a digital computer, through suitable digital-to-analog converters, so as to sweep the electron beam through a desired pattern and to expose the electron-sensitized surface on a target wafer which is appropriately disposed with respect to the beam. In electron image projection systems, a photocathode mask is illuminated with a suitable light source, typically ultraviolet light, so as to cause the emission of electrons in a pattern for acceleration toward and exposure of the electron-sensitized surface of the target wafer. In either system, a large accelerating electric field is imposed between the electron source and the target, to minmize any undesired excursion of the electrons in a direction perpendicular to the beam or projection. However, in either system, electrons are emitted with some component of velocity perpendicular to their desired trajectory, and consequently, some means of focusing the electron beam or image is generally provided.
Unfortunately, the electrical characteristics of a typical target holder for electron projection systems, and particularly for electron image projection systems, may cause certain distortion of the image formed on the target. Since image projection systems are more susceptible to this type of distortion than electron beam pattern generators, the problems and characteristics of an electron image projection system are more particularly described herein. It should be recognized, however, that the principles of focusing and the problem of target hold-down to be described are also directly applicable to electron beam pattern generators.
In a typical electron image projection system using a photocathode mask, an ultraviolet light source is mounted within a vacuum chamber, typically near the top thereof, and a mask support is provided to support the photocathode mask for direct and substantially uniform illumination by the light source. The target wafer on which the image is to be projected is mounted below the mask and is electrically biased at a high positive potential relative to the photocathode surface, so that electrons emitted from the photocathode will be rapidly accelerated to the target surface and will impinge thereon in the same pattern as they were emitted from the photocathode. However, since the electrons are emitted with some finite energy, and generally may be emitted in practically any direction, any emission velocity component parallel to the photocathode surface will result in lack of focus in the image produced at the target. To focus the image, a magnetic field of controlled uniform intensity may be created having a direction parallel to the desired path of the electrons. Thus, the undesired component, that is, the horizontal component of velocity of the electrons, is a component perpendicular to the magnetic focusing field.
As is well known, an electron moving perpendicularly to a magnetic field will follow a circular orbit. The radius of the orbit is given by the equation: R = MV/Bq, where R is the radius of the circular orbit, M is the mass of the electron, V is the velocity of the electron perpendicular to the uniform field, B is the flux density of the magnetic field, and q is the charge of the electron. The time required for an electron to make one circular orbit is equal to the circumference of that orbit divided by the electron velocity, or T = 2 .pi.R/V. Substituting for the radius R as given in the previous equation, it may be seen that the time for a circular orbit is 2 .pi.M/Bq. Thus, the time required for an electron to make a circular orbit is independent of the component of velocity perpendicular to the magnetic field. Consequently, if the magnetic flux density is properly selected, the electrons may be caused to follow a helical trajectory in which an integral number of orbits is completed before the electrons impinge on the target. Thus, the image is focused on the target irrespective of the horizontal component of velocity of particular electrons, although the image at various planes between the photocathode and the target will not necessarily be in focus.
It will be apparent that focusing as outlined above will be achieved without distortion of the image impinging on the target only if the equipotential surfaces between the photocathode and the target are truly parallel and flat surfaces, and if the axial magnetic field is perfectly uniform. In systems of the prior art, the mounting of both the photocathode and the target have been such that the equipotential surfaces at and near both the photocathode and the target are not perfectly flat surfaces. This has the undesired effect of distorting the image on the target, since, there will be components of the electric field in directions parallel to the photocathode and the target. Electrons exposed to these components will be accelerated parallel to the cathode and target.
As has been explained, electrons emitted from the photocathode with an undesired component of velocity parallel to the photocathode may be focused electro-magnetically in an ideal system having a perfectly uniform electric field. However, if such an undesired velocity component is obtained as a result of field distortion, focus cannot always be effected in the same manner. That is, some electrons will not complete one full orbit or an integral number of orbits before impinging upon the target. This effect, unless extreme, does not exhibit itself primarily as a lack of focus, because the electrons emitted from a particular area on the photocathode will each experience the same electric field profile, and therefore the same fractional orbit, before impinging on the target.
Rather than being out of focus at the target, the electrons emitted at a position radially displaced from a central axis of the system will impinge on the target at a laterally shifted position which is not directly under the area from which they were emitted. Since the electric field distortion normally becomes particularly severe adjacent to the edges of the target and the photocathode, the resulting pattern distortion is at a maximum near the edges of the pattern. While there will also be some loss in focusing at the edges, the primary effect of the distortion in the electric field is a so-called spiral or anisotropic distortion in the image projected onto the target.
In integrated circuit manufacturing, elements having a width of only a few ten thousandths of an inch are defined by a pattern which may be created by an electron image projection system having an edge definition in the range of 10 micro-inches or less. Consequently, any significant distortion, and particularly distortion between individual masks within a mask set, can cause the resulting circuit to be inoperative.
In addition to problems relating to distortion of the electron-accelerating field, target wafer holders in systems available heretofore have been unsuccessful in holding thin wafers of target material perfectly flat. Typically, in the prior art, a wafer is secured by its edges under a circumferential lip or a plurality of edge clamps which hold the wafer in position. Since the target material usually takes the form of thin wafers which are often warped and bowed, holding the wafers down by their edges does not always completely flatten them, and often contributes additional non-planar distortion.
There exists, therefore, an urgent need for a wafer supporting and securing means which will hold a wafer perfectly flat, will minimize the distortion of the electron accelerating field, and will thereby allow the projection of more accurate images for such purposes as the fabrication of high-accuracy masks and for micro-electronic circuitry fabrication from such masks. The present invention fulfills this need.